This invention relates to a device for adjusting the angle of Faraday rotation (Faraday rotator) and also to an optical attenuator using such a device. The Faraday rotation angle-adjusting device and optical attenuator according to the present invention are specifically used in optical transmission communication systems. The invention, in particular, improves the temperature dependence of Faraday rotation angle and narrows the scatter of variations among products in the angle of Faraday rotation with externally applied variable magnetic fields. Moreover, the invention reduces the variable magnetic field required to achieve a desired low-current characteristic or specific amount of attenuation (proportional to the angle of Faraday rotation).
Owing to the striking expansion of transmission capacities, there is a growing demand for high-density-wavelength multiplex transmission systems. This has accordingly increased the need for variable optical attenuators that dynamically adjust the quantity of light required for the systems. Applications for the attenuators include the control of light quantities for individual channels and simultaneous attenuation of multiplex light rays. Among those optical attenuators there is one type that utilizes magneto-optical effect. That type usually involves a layout in which a component capable of changing the angle of Faraday rotation a light beam is disposed between a polarizer and an analyzer. With one such component for changing the Faraday rotation angle, external magnetic fields are applied from two or more different directions to a single crystal of garnet having a Faraday effect so as to make the composite external field variable, whereby the Faraday rotation angle of the light that passes through the single crystal of garnet is controlled (Registered Japanese Patent 2,815,509).
To be more specific, the Registered Japanese Patent 2,815,509 discloses an optical attenuator which, while keeping a fixed magnetic field greater than the saturation magnetic field of a single crystal of garnet applied to the crystal in a direction parallel to the optical axis by means of a permanent magnet, applies a variable magnetic field to the crystal in a direction perpendicular to the optical axis by an electromagnet, thereby changes the composite magnetic field vector, and changes the direction of magnetization and hence the Faraday rotation angle of the single garnet crystal so that the quantity of light coupled to the fiber on the leaving side can be controlled.
Another method is known as a means of decreasing the temperature dependence of optical attenuators, which comprises applying external magnetic fields in the directions where the amount of change of the Faraday rotation angle due to the temperature dependence of the angle between the direction of magnetization of Faraday elements and the direction of the beam and the amount of change of the Faraday rotation angle due to the temperature dependence with the Faraday effect are different in code from each other and the absolute value of either amount is less than twice that of the other amount, whereby the changes of Faraday rotation angle with temperature are restricted. (Japanese Patent Application Kokai No. 11-249095).
With the foregoing in view, a plurality of optical attenuators were experimentally fabricated. The experiments presented a problem of wide scatter among the specimens of the temperature dependence values of the attenuation and the electromagnet field perpendicular to the optical axis required to attain the maximum attenuation. Another problem that arose was the requirement of a large variable magnetic field (and hence a large driving current) to achieve a desired attenuation (proportional to the angle of Faraday rotation).
Therefore, the solution of these problems in accordance with the present invention is to provide an optical attenuator that narrows the scatter of characteristics among devices, reduces the temperature dependence, and possesses good low-current characteristics.
Also, in view of the foregoing, we experimentally fabricated a plurality of optical attenuators and found that they show, in common, that an increase in the quantity of attenuation is accompanied with an increase in the polarization dependence loss (hereinafter abbreviated to xe2x80x9cPDLxe2x80x9d) up to more than one decibel at peak. The value is by far the greater than those of non-polarization-dependent optical isolators, the optical devices that similarly take the advantage of magneto-optical effect and are already in wide use with optical transmission systems. Typical PDL values of the non-polarization-dependent optical isolators are of the order of 0.1 dB.
A study of the difference revealed that, when birefringent elements are used as a polarizer and an analyzer, ordinary and extraordinary rays pass through a Faraday element along different paths and accordingly the distributions of magnetic fields that are applied to different portions of the Faraday element vary too. Factors that can subtly influence the magnetic field distributions are, for example, the direction of application of variable magnetic field, shape and size of the yoke of an electromagnet that produces the variable field, and the relative positions of the yoke and Faraday element.
FIG. 22 illustrates, by way of example, a conventional member for attaching a Faraday element to an optical attenuator. The member has a cutout 20 for receiving a yoke and an opening 15 formed in the center for the passage of a light ray. A Faraday element recess 21 is formed at the bottom of the cutout 20 in alignment with the opening 15, and a Faraday element (not shown) consisting of one or more garnet crystal plates is received in the recess. The Faraday element is secured in place with a thermosetting or ultraviolet-curing resin filled in resin-filling ports 16, 17 open to the cutout 20. Yokes 10, 10 of electromagnets are inserted on both sides of the opening 15 into the cutout 20 and are similarly fixed with a hardening resin. There is no means of positioning the inner ends of the yokes 10 of electromagnets, and the yokes simply secured with respect to the Faraday element fail to maintain a constant positional relationship. Moreover, the combined area of the yokes relative to the Faraday element is restricted. These and other factors are deemed responsible for the problem of increased PDL.
Therefore, another problem that the present invention is to solve are settled by the provision of an optical attenuator having favorable PDL characteristics.
The present invention provides a Faraday rotator in which magnetic field is applied to Faraday elements, optical axis of said Faraday elements being in the  less than 111 greater than  direction of single crystal of garnet, characterized in that
three single crystals of garnet of substantially the same thickness having a Faraday effect are used to form the Faraday elements and the Faraday elements are arranged in such a manner that a first magnetic field is applied to one of the Faraday elements, in the direction perpendicular to a plane in a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of a stereographic projection chart with the (xe2x88x921xe2x88x9212) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a second magnetic field is applied to the remaining two elements, in the direction perpendicular to plane in a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x92101) plane on the outermost circumference or a plane equivalent thereto in the chart. Each of said first and second magnetic fields may be a composite magnetic field formed by a pair of magnetic fields.
The present invention also provides a Faraday rotator in which both a magnetic field parallel to and a magnetic field perpendicular to the optical axis are applied to Faraday elements, said optical axis being in the  less than 111 greater than  direction of single crystal of garnet, characterized in that
three single crystals of garnet of substantially the same thickness having a Faraday effect are used to form the Faraday elements and the Faraday elements are arranged in such a manner that a first magnetic field is applied to one of the Faraday elements, in the direction perpendicular to a plane in a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x921xe2x88x9212) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a second magnetic field is applied to the remaining two elements, in the direction perpendicular to a plane in a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x92101) plane on the outermost circumference or a plane equivalent thereto in the chart.
The present invention also provides an optical attenuator including a polarizer and an analyzer disposed, respectively, before and after a plurality of Faraday elements, in which a variable magnetic field is applied to Faraday elements, in such manner that the variable magnetic field can change the angle of Faraday rotation of a light beam and control the quantity of light transmitted, said optical axis being in the  less than 111 greater than  direction of single crystal of garnet, characterized in that three single crystals of garnet of substantially the same thickness having a Faraday effect are used to form Faraday elements and the Faraday elements are arranged in such a manner that a variable magnetic field is applied to one of the Faraday elements, in the direction perpendicular to planes over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x921xe2x88x9212) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a variable magnetic field is applied to the remaining two elements, in the direction perpendicular to planes over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x92101) plane on the outermost circumference or a plane equivalent thereto in the chart.
The variable magnetic field may be a composite magnetic field applied from a pair of magnetic fields at least one of which is variable.
The present invention also provides an optical attenuator including a polarizer and an analyzer disposed, respectively, before and after a plurality of Faraday elements, in which both a magnetic field parallel to and a magnetic field perpendicular to the optical axis are applied to the Faraday elements, one of the magnetic fields being fixed and the other being variable, so that the composite magnetic field thereof can change the angle of Faraday rotation of a light beam and control the quantity of light transmitted, said optical axis being in the  less than 111 greater than  direction of single crystal of garnet, characterized in that
three single crystals of garnet of substantially the same thickness having a Faraday effect are used to form Faraday elements and the Faraday elements are arranged in such a manner that a variable magnetic field is applied to one of the Faraday elements, in the direction perpendicular to planes over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x921xe2x88x9212) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a variable magnetic field is applied to the remaining two elements, in the direction perpendicular to planes over a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (xe2x88x92101) plane on the outermost circumference or a plane equivalent thereto in the chart.
In the Faraday rotator and optical attenuator defined above, preferably the magnetic field parallel to the optical axis is a fixed magnetic field generated by permanent magnets and the magnetic field perpendicular to the optical axis is a variable magnetic field generated by electromagnets.
According to the invention, the Faraday rotator is composed of a combination of three Faraday elements of specific orientation, and not only the temperature dependence of Faraday rotation angle is reduced but also the scatter of Faraday rotation characteristics is controlled.
To be more specific, the Faraday rotator and attenuator according to the present invention are limited in temperature variation of the attenuation (that depends on the Faraday rotation angle) and, moreover, the scatter of the attenuation-temperature variation among the specimens is narrow. Furthermore, the scatter of the peak current value (corresponding to the variable magnetic field required to achieve the maximum change in the Faraday rotation angle) among the specimens is small and, in addition, low-current characteristics can be attained (that is, the driving current to generate a variable magnetic field necessary for obtaining a certain amount of attenuation or Faraday rotation angle may be small).
Another Means of Solving the Problems
The present invention also provides an optical attenuator which controls the angle of Faraday rotation of a light beam that passes through a single crystal of garnet having the Faraday effect by applying two external magnetic fields, fixed and variable, from two different directions, characterized in that at least one garnet crystal having the Faraday effect is used as a Faraday element, and a member for holding the Faraday element in place has a stopper to position the front ends of yokes of electromagnets that apply the variable magnetic field to and around the holder, with respect to the direction of field application.
The invention also provides an optical attenuator as defined above characterized in that the member for holding the Faraday element in place has a pair of positioning grooves to position the front ends of yokes of electromagnets that apply the variable magnetic field to and around the holder, with respect to the direction of light beam.
The invention further provides an optical attenuator according to claim 1 characterized in that the yokes of the electromagnets that apply the variable magnetic field have a front end plane each perpendicular to the direction of the variable field with a cross sectional area no less than 1.7 times that of the plane of the Faraday element perpendicular to the direction of the variable field.
To be more concrete, the invention provides an optical attenuator comprising a member formed with a first groove extending across the optical axis and also formed with an opening open to the first groove along the optical axis, a Faraday element disposed in the first groove in alignment with the optical axis, said member having a pair of second grooves formed on both sides of, and close to, the Faraday element, said second grooves extending across the first groove and in the direction normal to the optical axis, and a pair of electromagnets that produces a variable magnetic field, said magnets having yokes the ends of which are fitted in the pair of second grooves on both sides of the Faraday element, the bottoms of the second grooves serving as a stopper for positioning the front ends of the yokes.